<!DOCTYPE html>
<html>
<head>
    <meta charset="UTF-8">
    <title>Document</title>
    <style>
        
    </style>
</head>
<body>
    <input type="text" id="txt1">
    <select id="computed">
        <option value="+"> + </option>
        <option value="-"> - </option>
        <option value="*"> * </option>
        <option value="/"> / </option>
        <option value="%"> % </option>
    </select>
    <input type="text" id="txt2">
    <input type="button" value="=" id="action">
    <input type="text" id="result" readonly>
</body>
<script>
    
    // 简易计算器

</script>
</html>